Contemporary art was invigorated by artists like Marcel Duchamp who put ordinary objects on display and called them art. Ian Shadwell argues that controversial British artist Damien Hirst - who built his career and his fortune on this idea by preserving dead animals and exhibiting them in galleries - has put the Found Art movement at risk by insisting a packet of pencils, taken from one of his new works, is irreplacible and the teenager who stole them should be arrested for theft.